Reactive polyurethane hotmelts are a fast-growing product group within polyurethane applications in the adhesives field. They are synthesized using preferably linear polyester- and/or polyetherpolyols in combination with an excess of polyisocyanates, preferably diisocyanates.
The advantages of this class of product lies above all in the absence of solvent, the possibility of applying the products hot at relatively low viscosities, of nevertheless obtaining high strengths and, after a relatively short time, owing to further reaction with moisture, of obtaining adhesive bonds having very high thermal stability, well beyond the application temperatures, and excellent solvent resistances.
Essential to the good profile of properties of the reactive polyurethane hotmelts is their ability to develop strengths very rapidly on cooling, which allows the joined parts to be handled immediately after joining.
Responsible for the development of the initial strengths are, as in the case of all hotmelts, only physical phenomena, since in a period of seconds to minutes it is not yet possible for any major chemical events to unfold. These physical events are in particular the strong, continuous rise in viscosity as a result of temperature reduction, further superimposed in some cases by a recrystallization effect, which constitutes a jump in the increase in strength.
One effective way of describing the events accompanying the cooling of polymer melts is to record the changes in the viscoelastic properties of the melts against the temperature. A particular possibility arising here is that of studying the events over the temperature range which coincides with the ranges that are of interest at that time.
In vibration experiments, i.e. where the deformation changes sinusoidally, the relaxation behaviour of a viscoelastic substance is manifested in a phase shift δ between the applied deformation and the resulting strain (or torque). According to definition, the following is the case: for purely elastic fluids the phase angle δ=0° and for purely viscous liquids an angle of δ=90° is measured. Characterizing variables are the storage modulus and loss modulus G′/G″ (Pa), the complex viscosity η* (Pas) and the phase angle δ (°).
Use is made in particular of the variable of the storage modulus G′ in the development of adhesives, under the concept of what is termed the Dahlquist criterion or of the PSA band (pressure sensitive adhesives). In the literature, the storage modulus range G′ from 5×104 to 5×105 Pa is assigned to the Dahlquist criterion. The Dahlquist criterion, in other words the presence of a storage modulus in the range from 5×104to 5×105 Pa, denotes the ability of polymers to bond with themselves, with other polymers and with other substrates.
The model developed by Dahlquist (C. A. Dahlquist, Proc. Nottingham Conf. On Adhesion, Maclaren & Sons Ltd., London 1996, Part III, Chapter 5; or else in A. J. Frank, Adhesives Rheology, brochure Rheometrics Present at Afera Congress in Chester, Sep. 24, 1992) starts initially from the purely mechanical conception that, before the various physical adhesion mechanisms (dipole interactions, hydrogen bonds, van der Waal forces, diffusion of chains) can become effective, the materials must be brought into intimate contact to allow these forces (with a range of just a few angstroms) to be active at all. It is illuminating that this contact problem becomes greater as a result of high storage moduli. In the case of adhesives, the lower limit is set via an inadequate cohesive strength.
When some typical reactive polyurethane hotmelt systems are viewed from this standpoint, the cases described below are observed.
When crystalline polyols are used, the storage modulus G′ extends to just above the recrystallization temperature in the region of <1000 Pa; in other words, the melt at this point has no cohesive strength at all, and adherends must be held mechanically. The Dahlquist criterion is then traversed within a temperature range of a few ° C., in order to build up storage moduli of >106 Pa immediately, which correspond to forces so high that they no longer allow any repositioning of the substrates to be bonded at all.
Hotmelt systems based on crystalline polyesterpolyols, as described for example in EP-A 0 354 527, exhibit very low viscosities above the recrystallization temperature, which although they allow effective wetting of the surface are at this point unable to develop any cohesive strengths whatsoever. Only as recrystallization begins are initial strengths developed, albeit then high ones.
When polyols which are liquid at room temperature are used, the Dahlquist criterion is not met in the room temperature range; that is, the substrates which are to be bonded with adhesives of this kind must be fixed mechanically until a chemical reaction with atmospheric moisture ensues.
FIG. 1 shows exemplarily the course of the storage modulus as a function of the temperature for the first (comparative examples 1 and 2) and the second (comparative example 3) case.
Within the art, attempts are made, by combining crystalline polyols, polyols which are liquid at room temperature (glass transition temperatures Tg<20° C.) and amorphous polyols with higher glass transition temperatures (Tg>20° C.), to optimize the Dahlquist range, i.e. to obtain a polymer melt having PSA properties which allows a certain repositioning of the substrates to be bonded, but which owing to the PSA properties already has sufficient strength to hold this position. For instance, “Shaping Reactive Hot Melts Using LMW Copolyesters”, Adhesives Age, November 1987, p. 32 ff. describes reactive polyurethane hotmelts comprising crystalline polyesters, polyesters which are liquid at room temperature, and amorphous polyesters.
EP-A 0 340 906 discloses reactive polyurethane hotmelts composed of a mixture of two polyurethane prepolymers, the first prepolymer being prepared from an amorphous polyol having a glass transition temperature >20° C. and the second prepolymer from a polyol which is liquid at room temperature (Tg<20° C.).
EP-A 0 511 566 describes an NCO-reactive polyurethane hotmelt adhesive composition obtainable from a mixture of a polyfunctional polyol component which is of high viscosity or liquid at room temperature and a polyfunctional polyol component which is crystalline at room temperature.
Relatively high concentrations of high-Tg polyols, however, cause embrittlement of the adhesive films and a sharp rise in viscosity, which may adversely affect the wetting of the surface.